1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device having a semi-super junction structure.
2. Background Art
Vertical MOSFET (metal-oxide-semiconductor field effect transistor) devices are used for power electronics applications. The ON resistance of such a MOSFET device greatly depends on the electric resistance of its conduction layer (drift layer). The electric resistance of the drift layer is determined by its doping concentration. The ON resistance can be decreased by increasing the doping concentration. However, increase of the doping concentration results in decreasing the breakdown voltage of a pn junction formed by the drift layer and the base layer. Hence it is difficult to substantially increase the doping concentration. Thus there is a tradeoff between device breakdown voltage and ON resistance. Improving this tradeoff is a significant challenge for realizing power semiconductor devices with low power consumption and high breakdown voltage.
As an example MOSFET device improving this tradeoff, JP-A 2004-119611 (Kokai) discloses a structure known as a “super junction structure” in which p-type column regions and n-type column regions shaped like vertically elongated strips are laterally and alternately juxtaposed in the drift layer. In the super junction structure, a non-doped layer can be artificially produced by nearly equalizing the concentration of dopant contained in the p-type column region and in the n-type column region. Thus, while supporting high breakdown voltage, a current is allowed to flow through the highly doped n-type column region, and thereby low ON resistance beyond the material limit can be realized.
The breakdown voltage of such a MOSFET having a super junction structure can be further enhanced by increasing the layer thickness of the p-type column region and the n-type column region. However, if the repetition pitch of the p-type column region and the n-type column region is left unchanged, thickening the column regions results in increasing the aspect ratio of the p-type column region and the n-type column region, and the process for forming them becomes difficult. On the other hand, high breakdown voltage can be achieved also by decreasing the concentration in the p-type column region and the n-type column region and thickening the column regions to increase the repetition pitch with the aspect ratio kept constant. However, in this case, although high breakdown voltage is obtained, the ON resistance substantially increases because of low doping concentration in the n-type column region, where current flows at turn-on time.
In this respect, Japanese Patent No. 3634848 discloses a semiconductor device in which an n-type layer is inserted on the collector side of the p-type column region and the n-type column region. Thus the breakdown voltage is supported by both the super junction section and the n-type layer, thereby increasing the breakdown voltage of the device. In this case, the breakdown voltage of the device can be increased without requiring substantial changes in the current process for forming the super junction structure. The proportion of the p-type column region and the n-type column region occupying the drift layer favors the tradeoff between breakdown voltage and ON resistance as compared with the uniform structure of the conventional n-type drift layer.